Modern antilock brake systems can be thought of as electronic/hydraulic pumping of the brakes for straight-line stopping under panic conditions. Good drivers have always pumped the brake pedal during panic stops to avoid wheel lock up and the loss of steering control. Antilock brake systems simply get the pumping job done much faster and in a much more precise manner than the fastest human foot. When the brake pedal is pumped or pulsed, pressure is quickly applied and released at the wheels. This is called pressure modulation which works to prevent wheel locking and it can modulate the pressure to the brakes as often as fifteen times per second. By doing this friction between the tires and the road is maintained and the vehicle is able to come to a controllable stop.
The manoeuvrability of the vehicle is reduced if the front wheels are locked, and the stability of the vehicle is reduced if the rear wheels are locked. A locked tire skids on pavement and has poor traction. This condition allows for 100% tire slip, whereas a tire rolling freely has a slip of nearly 0%. Slip is the difference between the actual speed of the vehicle and the speed of the tire’s tread as it rotates on the pavement. Antilock brake systems control the slip rate of the wheels to ensure maximum grip force, or traction, at the tires. It is the traction of the tires that actually stops the vehicle; therefore, ABS can improve braking and handling by controlling the brake fluid pressure at each wheel to attain the target slip rate at that wheel. Although ABS prevents complete wheel lockup, it allows some wheel slip in order to achieve the best braking possible.
Hydraulic components of ABS
An accumulator is used to store hydraulic fluid to maintain high pressure in the brake system and to provide residual pressure for power-assisted braking.
Antilock hydraulic control valve assembly:
This assembly controls the release and application of brake system pressure to the wheel brake assemblies.
The booster pump is an assembly of an electric motor and pump. The booster pump is used to provide pressurized hydraulic fluid for the ABS.
Booster/Master cylinder assembly:
The booster/ master cylinder assembly, sometimes referred to as the hydraulic unit, contains the valves and pistons needed to modulate hydraulic pressure in the wheel circuit during ABS operation.
Different than a pressure accumulator, fluid accumulators temporarily store brake fluid removed from the wheel brake units during an ABS cycle. This fluid is then used by the pump to build pressure for the brake hydraulic system.
Hydraulic control unit:
This assembly contains the solenoid valves, fluid accumulators, pump, and an electric motor.
This two-position valve is also controlled by the ABS control module and is open only in the ABS mode. When open, pressurized brake fluid from the booster circuit is directed into the master cylinder (front brake) circuits to prevent excessive pedal travel.
The modulator unit controls the flow of pressurized brake fluid to the individual wheel circuits. Also known as the hydraulic actuator, hydraulic power unit, or electrohydraulic control valve.
The solenoid valves are located in the modulator unit and are electrically operated by signals from the control module. The control module switches the solenoids on or off to increase, decrease, or maintain the hydraulic pressure to the individual wheel units.
Valve block assembly:
The valve block assembly attaches to the side of the booster/master cylinder and contains the hydraulic wheel circuit solenoid valves. The control module controls the position of these solenoid valves. The valve block is serviceable separate from the booster/master cylinder but should not be disassembled. An electrical connector links the valve block to the ABS control module.
Wheel circuit valves:
Two solenoid valves are used to control each circuit or channel. One controls the inlet valve of the circuit, the other controls the outlet valve. When the inlet and outlet valves of a circuit are used in combination, pressure can be increased, decreased, or held steady in the circuit.
Information derived from: “Automotive technology: A systems approach. 5th edition. Jack Erjavec”